This invention relates to a process for converting an oxygenate to high purity olefins by contacting the oxygenate with a molecular sieve catalyst. More particularly, the invention relates to a process for converting a methanol or methanol-water mixture to polymer-grade propylene by contacting the feed with zeolite or silicoaluminophosphate catalysts.
High purity olefins such as propylene have traditionally been produced through the process of steam and/or catalytic cracking. Because of the limited availability and high cost of petroleum sources, the cost of producing high purity olefins from such petroleum sources has been steadily increasing. Curtailment in the availability of inexpensive petroleum raw materials threatens the supply of high purity olefins such as polymer-grade propylene. Polymer-grade propylene is used in the production of many types of plastics such as polypropylene.
The search for alternative materials for high purity olefin production has led to the use of oxygenates such as alcohols, and more particularly to methanol and higher alcohols or their derivatives. These alcohols may be produced by fermentation or from synthesis gas. Synthesis gas can be produced from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal wastes, or any organic material. Thus, alcohol and alcohol derivatives may provide non-petroleum based feeds for high purity olefin production.
Molecular sieves, such as crystalline zeolite catalysts, are known to promote the conversion of oxygenates to olefin-containing hydrocarbon mixtures. U.S. Pat. No. 4,025,575 and U.S. Pat. No. 4,083,889 disclose processes for conversion of methanol and/or methyl ether to olefin-containing products using ZSM-5-type zeolite catalysts.
Chang et al., U.S. Pat. No. 4,724,270, describe a process for converting methanol feedstocks to aromatic hydrocarbons using zeolite catalysts (having a silica-to-alumina ratio of at least 12) at a temperature of at least 725.degree. C. (1337.degree. F.). By conducting the reaction at elevated temperatures, zeolite dehydroxylation can occur. This can lead to zeolite decomposition to amorphous silica and alumina.
U.S. Pat. No. 4,433,189 to Young teaches conversion of methanol to light olefins over zeolite catalysts having a silica-to-alumina molar ratio of at least 12, and at a temperature of from about 200.degree. C. (392.degree. F.) to 500.degree. C. (932.degree. F.). The Young patent does not teach conversion of methanol to products such as polymer-grade propylene.
U.S. Pat. No. 4,677,243 to Kaiser teaches the formation of light olefins using silicoaluminophosphate catalysts at a temperature of from about 200.degree. C. (392.degree. F.) to 700.degree. C. (1292.degree. F.). Kaiser does not teach the formation of high purity olefins or polymer-grade propylene.
High purity olefins are generally recognized by those skilled in the art as products, excluding methane, which have a paraffin-to-olefin weight ratio of less than or equal to about 0.05. Purification of high purity olefins traditionally requires removal of low-level impurities which interfere with polymerization catalysis, or which interfere with other processes requiring high-purity reactants. Low-level contaminants include, but are not limited to, polar molecules, oxygenates such as water, alcohols, carbon monoxide, carbon dioxide, carbonyl sulfide (COS), oxygen, and other miscellaneous contaminants including hydrogen sulfide, mercaptans, ammonia, arsine phosphine, chlorides, etc. Low-level contaminants are removed by a variety of processes including, but not limited to adsorption and fractional distillation. Lighter or heavier hydrocarbon molecules having fewer or more carbon atoms than the desired olefin product must also be removed. These hydrocarbons are typically removed by fractional distillation techniques.
One such high purity olefin is polymer-grade propylene. Polymer-grade propylene is required for the production of polypropylene and useful for the production of other propylene derivatives. Polymer-grade propylene is characterized by very low concentrations of impurities, including low levels of paraffins (saturated hydrocarbons) such as propane, ethane, and butane.
Commercial chemical-grade propylene, unlike polymer-grade propylene, is characterized by higher concentrations of saturated hydrocarbons. Propane is the most difficult of the saturated hydrocarbons to remove from propylene, due in large part to the proximity of the boiling points for propane and propylene. Typical polymer-grade propylene purities range from 95% to 99.5% propylene, and are more preferably above 99%. This degree of purity corresponds to propane-to-propylene ratios of about 0.05 to about 0.01 or lower. Before the teachings of the present invention, this low propane level could only be practically achieved through the use of the well known art of fractional distillation. The fractional distillation scheme employed for effecting the difficult separation of propane from propylene is called "superfractionation." However, superfractionation requires a substantial investment in facilities and consumes copious amounts of energy. Alternative means for removing paraffin impurities, such as membrane techniques and adsorbent techniques, are as costly as the superfractionation techniques. The present invention teaches a means for producing high purity olefins having the required range of paraffin-to-olefin ratios, without the need to resort to superfractionation or other expensive purification techniques. According to the teachings of the present invention, a superfractionator is not required, thereby significantly reducing the cost of producing high purity olefins such as polymer-grade propylene.
These and other disadvantages of the prior art are overcome by the present invention, and a new, improved process for selectively converting oxygenates to high purity olefins is provided.